Device and method for information about PLMN ID

ABSTRACT

A mobile station for use in a Radio Access Network, RAN, connected to a core network in which there is one or more Public Land Mobile Networks, PLMNs. The mobile station is arranged to select one of said PLMNs, and is arranged to communicate with its RAN by means of Radio Link Control, RLC, data blocks. The mobile station is arranged to include the identity, ID, of its selected PLMN in an RLC data block, and to inform the RAN of the presence of the ID of the chosen PLMN in the RLC data block by means of including a reserved or pre-defined value in the length indication field of the RLC data block.

This application is the U.S. national phase of International ApplicationNo. PCT/SE2012/051000 filed 21 Sep. 2012 which designated the U.S. andclaims priority to U.S. Provisional Application No. 61/538,216 filed 23Sep. 2011, the entire contents of each of which are hereby incorporatedby reference.

TECHNICAL FIELD

The present invention discloses a device and a method for letting amobile station send information about a chosen PLMN.

BACKGROUND

If a communications system includes a Core Network, CN, and a RadioAccess Network, RAN, and both the CN and the RAN support so calledmultiple-operators core network, FULL-MOCN, i.e. more than one PLMN inthe CN, then, as part of the process of connecting a mobile station, MS,which also supports FULL-MOCN to its preferred Public Land MobileNetwork, PLMN, in the core network, the radio access network, RAN, whichconnects the mobile station to the core network, CN, needs to be awareof the ID of the preferred PLMN, which is at present a 4 octet longfield.

At present, there is no mechanism for an MS to indicate its preferredPLMN ID to the RAN when accessing the PS domain, i.e. when connectingthe MS to the CN, although this is needed in order to enable FULL-MOCNin a communications system.

One method for solving this which has been suggested in 3GPP is tointroduce a new Radio Link Control/Medium Access Control, RLC/MAC,message for sending a PLMN ID from the MS to the RAN.

This method might work, but would necessitate the introduction of a newmessage, and would also increase the load on the “air interface”, i.e.the interface between the MS and the RAN.

Another proposed method is to include the PLMN ID in Non-Access Stratum,NAS, messages sent to the CN. Since NAS messages are tunneled via theRAN on their way to the CN and are thus transparent to the RAN, thisimplies two significant drawbacks with this approach:

-   -   1. Extra signaling procedures, new and/or a re-definition of        existing messages between the CN and the RAN are needed.    -   2. The RAN needs to more or less randomly select one of the        PLMNs to decode this NAS message. This PLMN could then very well        “hijack” the mobile station by ignoring the PLMN ID that was        included from the beginning, i.e. by the MS.

SUMMARY

It is an object of the invention to obviate at least some of thedisadvantages mentioned above, and to provide a solution for how the IDof a mobile station's selected PLMN can be conveyed to a RAN to whichthe mobile station is connected.

This object is obtained by means of a mobile station for use in a RadioAccess Network, a RAN, the RAN being connected to a core network inwhich there is one or more Public Land Mobile Networks, PLMNs.

The mobile station is arranged to choose one of said PLMNs whenaccessing a Packet Switched, PS, domain in the RAN, and the mobilestation is also arranged to communicate with the RAN by means of RadioLink Control, RLC, data blocks.

The mobile station is arranged to include the identity, ID, of itschosen PLMN in an RLC data block, and to inform the RAN of the presenceof the ID of the chosen PLMN in the RLC data block by means of includinga reserved or pre-defined value in the length indicator field of the RLCdata block.

In embodiments, the mobile station is arranged to include the ID of itschosen PLMN after the last Upper Layer PDU of the RLC data block.

In embodiments, the mobile station is arranged to use an LI value of 123as the reserved or pre-defined value.

In embodiments, the mobile station is arranged to use different RANmodes and being arranged to use different reserved or pre-defined valuesto indicate the presence of the ID of a chosen PLMN depending on the RANmode used.

In embodiments, the mobile station is arranged to use different reservedor pre-defined values for RAN modes GPRS and EGPRS.

The object stated above is also obtained by means of a Control Nodewhich is arranged to be part of a Radio Access Network, a RAN, which isconnected to a Core Network, CN, which comprises one or more Public LandMobile Networks, PLMNs.

The Control Node is arranged to receive Radio Link Control, RLC, datablocks from a mobile station in a cell in the RAN, and the Control Nodeis arranged to inspect the Length Indicator field in said RLC datablocks, and to identify a reserved or pre-defined value which indicatesthat the RLC data block comprises the ID of one of said PLMNs which hasbeen selected by the UE, and to find and extract said ID in the RLC Datablock, and to forward the ID to a node in the RAN or in the CN.

In embodiments, the node to which the PLMN ID is forwarded by theControl Node is an SGSN.

In embodiments, the Control Node is arranged to find said PLMN ID afterthe last Upper Layer PDU of the RLC data block in which the PLMN ID isincluded.

In embodiments, the Control Node is arranged to identify an LI value of123 as the reserved or pre-defined value.

In embodiments, the Control Node is arranged to handle more than one RANmode, and being arranged to handle different reserved or pre-definedvalues which indicate the presence of the ID of a chosen PLMN, dependingon RAN mode.

In embodiments, the Control Node is arranged to handle differentreserved or pre-defined values which indicate the presence of the ID ofa chosen PLMN for the RAN modes GPRS and EGPRS.

In embodiments, the Control Node is a GSM Base Station Controller, aBSC, arranged to be part of a Base Station System, BSS.

In embodiments, the Control Node is an LTE eNodeB.

In embodiments, the Control Node is a WCDMA RNC, Radio NetworkController.

As can be understood from the above, a principle of the invention is tolet the mobile station include the selected, i.e. preferred, PLMN ID inan RLC data block, however without having to be included in the upperPDUs, i.e. LLC PDUs (Logical Link Control Physical Data Units), thatcontain the actual NAS message, as would be the case if e.g. the PLMN IDwas included in a NAS message.

The inclusion of the PLMN ID in an RLC data block is, as shown above,achieved by re-using the length indication mechanism, which has hithertobeen used mainly for letting the RLC receiving entity be able tocorrectly re-assemble the various segments of received upper layer PDUs,i.e. LLC PDUs. More specifically, the invention shows the use of areserved or pre-defined value of the length indicator in an RLC datablock in order to indicate the presence of a PLMN ID which identifiesthe mobile station's preferred PLMN in the RLC data block. The actualPLMN ID is then suitably but not necessarily, included as the four lastoctets of that RLC data block, suitably immediately following the lengthindicator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, withreference to the appended drawings, in which

FIG. 1 shows a schematic view of a communications system, and

FIG. 2 shows a block diagram of a User Equipment, and

FIG. 3 shows a block diagram of a Base Station Controller, and

FIG. 4 shows a flow chart of a method for use in a User Equipment, and

FIG. 5 shows a flow chart of a method for use in a Control Node.

DETAILED DESCRIPTION

Embodiments of the present invention will be described more fullyhereinafter with reference to the accompanying drawings, in whichembodiments of the invention are shown. The invention may, however, beembodied in many different forms and should not be construed as beinglimited to the embodiments set forth herein. Like numbers in thedrawings refer to like elements throughout.

The terminology used herein is for the purpose of describing particularembodiments only, and is not intended to limit the invention.

FIG. 1 shows an overview of a communications system 100 which comprisesa Core Network, CN, 101, and a Radio Access Network, RAN, 102. The Ranwill in FIG. 1 and below be described with examples from GSM systems, sothat the RAN comprises a Base Station Subsystem, BSS 107, which in turncomprises a Base Transceiver Station 108 and a Base Station Controller,BSC 109. The combination of a BTS and a BSC is used in GSM systems, andit should be pointed out that reference is made herein to thiscombination by way of example only; the invention can also be applied inother kinds of systems, e.g. LTE systems, in which case the functiondescribed herein of the BSC 109 is located in an eNodeB, and in a WCDMAsystem the function would be located in Radio Network Controller, anRNC. A generic term for these nodes as described herein is “a controlnode”. However, the function of the control node will be described belowand in the drawings with reference to a BSC as part of a BSS togetherwith a BTS.

The CN 102 is a so called multi-operator core network, e.g. a so calledFULL-MOCN network, i.e. the CN 102 comprises one or more Public LandMobile Networks, PLMNs, and in the case of two or more PLMNs, the PLMNSare usually from two or more operators. This can also be seen asmultiple PLMNs sharing one and the same RAN 102.

FIG. 1 shows a first 103 and a second 104 PLMN in FIG. 1, as well as anNth PLMN 105, in order to indicate that there may be N PLMNs in the CN102. In addition, two SGSNs 106 and 111 are shown in FIG. 1, where theSGSN 106 is shared by PLMNs 103 and 104, so called Gateway Core NetworkGWCN, while the SGSN 111 is exclusive for the PLMN 105. Both of thesecases are possible.

As shown in FIG. 1, the CN 101 is connected to the RAN 102, and the RAN102 may accommodate a number of mobile stations, MSs, one of which isshown as 110 in FIG. 1. The MS 110 is arranged to select one of thePLMNs in the CN 101 that share the RAN in 102, e.g. when the MS accessesthe Packet Switched, PS, domain, e.g. when attaching to the CN 102and/or the RAN 101. The fact that the MS 110 is arranged to select or“prefer” a PLMN in this way can, for example, be due to the fact thatthe MS 110 is arranged to support FULL-MOON systems, sometimes alsoreferred to as “shared networks”.

The ID of the MS's preferred PLMN is conveyed from the MS 110 to the RAN102, and from there to the CN 102. In more detail, the MS 110 isarranged to communicate with the RAN 102 by means of so called RLC datablocks, and to include the ID of the preferred PLMN in an RLC datablock, as well as to use the RLC data block to inform the RAN of thepresence of the ID of the preferred PLMN in the RLC data block.

The RAN is informed by the MS 102 of the presence of the ID of thepreferred PLMN in the RLC data block by means of the so called LengthIndicator, LI, field in the RLC data block in that a reserved orpre-defined value of the LI field in the RLC data block indicates to theRAN that the RLC data block comprises the preferred PLMN ID, and the RANis then arranged to find the preferred PLMN ID in the RLC data block,and to forward it to the CN 102, where the MS is connected to thepreferred PLMN.

The presence of the ID of the preferred PLMN is thus conveyed by meansof a reserved or pre-defined value in the LI field; the actual PLMN IDas such can be included in the RLC data block in various places, forexample as the four last octets of the RLC data block in which thereserved or pre-defined value is included in the LI field. Anotherexample of how to include the PLMN ID in the RLC data block would be tolet the LI field be immediately followed by an octet that contains theso called PLMN Index field, as specified in 3GPP TS 44.060 v11.2.0Rel-11. The PLMN ID selected by the MS can also be included after thelast Packet Data Unit, PDU, in the RLC data block.

As mentioned, suitably, the MS 110 will include the PLMN ID and theindication of its presence in an RLC Data Block when accessing a PacketSwitched, PS, domain in the RAN 102.

Also suitably, the MS 110 is arranged to handle more than one kind ofRAN mode, and to use different reserved or pre-defined values toindicate the presence of the ID of a chosen PLMN for different RANmodes. For example, the MS 110 can be arranged to use different reservedor pre-defined values for GPRS and EGPRS systems, e.g. 62 for GPRS and123 for EGPRS.

As can be understood, suitably the MS does not include the PLMN ID inevery RLC data block it sends to the RAN; suitably the PLMN ID isincluded by the MS when the MS has data to send with a foreign or arandom TLLI, Temporary Logical Link Identifier, and the data that the MSthen sends with a foreign or random TLLI is suitably a registrationmessage, e.g. ATTACH or ROUTING AREA UPDATE REQUEST.

FIG. 2 shows a schematic block diagram of the MS 110: as can be seen,the MS 110 comprises an antenna unit 10, which is used as both atransmit and receive antenna. In addition, the MS 110 comprises areceive unit Rx 11, and a transmit unit, Tx 12. The function of the MS110 in general, as well as the function of the receive and transmitunits is controlled by a control unit 13. In addition, the MS 110 alsocomprises a memory unit 14, which can be used both to store informationand to store executable code which is used when operating the MS 110.The information which is stored in the memory unit 14 can, for example,be the ID of the PLMN or PLMNs which the MS 110 should choose andindicate as its preferred PLMN in a FULL-MOON system. The reserved orpredefined value or values of the LI field which indicate the presenceof a PLMN ID in an RLC data block is/are also preferably stored in thememory unit 14.

The ID of the MS's preferred PLMN is placed in an RLC data block by thecontrol unit, which is also the case of the pre-defined or reservedvalue which indicates the presence of the PLMN ID in the RLC data block.The RLC data block is then transmitted by the transmit unit 12, throughthe antenna unit 10, as controlled by the control unit 13.

It is also the control unit 13 which enables the MS 110 to use differentRAN Modes, such as GPRS and EGPRS.

Thus, the MS 110 transmits its choice of PLMN ID to the RAN 102, whereit is extracted from the RLC Data block and forwarded to the CN 101. Inmore detail, the MS 110 transmits its RLC data blocks to the controlnode of its cell; in the example shown in FIG. 1 with the control nodeas such being part of a so called BSS, Base Station System, shown inFIG. 1 as 107. The BSS 107 comprises a Base Transceiver Station, BTS108, and a Base Station Controller 109. Suitably, it is the BSC 109which uses the predefined or reserved LI field indicator value torecognize that an RLC data block comprises a PLMN ID, and “extracts” thePLMN ID from the RLC data block.

Thus, the BSC 109 “inspects” the LI field in the RLC data blocks, and ifit identifies a reserved or pre-defined value which indicates that theRLC data block comprises the ID of a PLMN, the BSC 109 then finds andextract the PLMN ID in the RLC Data block, and forwards the PLMN ID toanother node in the RAN 102 or in the CN 101. Suitably, this other nodeis a so called SGSN 106, 111, as shown in FIG. 1, which is in the CN101.

Since the BSC 109 is the node in the RAN that finds and extracts thePLMN ID, the BSC 109 is arranged to look for the PLMN ID in places inthe RLC data blocks corresponding to those places in which the MS 110 isarranged to place the PLMN ID, as described earlier in this text. Thus,in embodiments, the BSC 109 is arranged to find the PLMN ID as the fourlast octets of the RLC Data block in which the reserved or pre-definedvalue is included in the LI field, as an alternative or complement towhich the BSC 109 is arranged to find the PLMN ID immediately after theLI field, by means of an octet that contains the so called PLMN Indexfield, as specified in 3GPP TS 44.060 v11.2.0 Rel-11. Alternatively oras a complement, the BSC 109 is also arranged to find the PLMN IDselected by the MS after the last Packet Data Unit, PDU, in the RLC datablock in which the reserved or predefined value is found.

Suitably, the BSC 109 is arranged to handle more than one kind of RANmode, and is then arranged to handle different reserved or pre-definedvalues which indicate the presence of the ID of a chosen PLMN, dependingon different RAN systems. For example, the BSC 109 can in embodiments bearranged to handle different reserved or pre-defined values whichindicate the presence of the ID of a chosen PLMN for GPRS and EGPRSsystems, e.g. the value of 62 for GPRS and 123 for EGPRS.

FIG. 3 shows a schematic block diagram of the BSC 109: as can be seen,on this level of abstraction, there are similarities between the MS 110and the BSC 109. As shown in FIG. 3, the BSC 109 comprises an I/O unit20 for communication with other nodes in the system 100, e.g. the BTS108 and nodes in the CN 101, suitably one or more SGSN, such as theSGSNs 106 and 111 shown in FIG. 1. The I/O unit can comprise antennaunits, for wireless communication, but will also usually comprisepossibilities for “landline” connections with the BTS 108 and the CN101, e.g. the SGSN(s) 106, 111, in the CN.

In addition to the I/O-unit 20, the BSC 109 also comprises a receiveunit Rx 21, and a transmit unit, Tx 22. The function of the BSC 109 ingeneral, as well as the function of the receive and transmit units iscontrolled by a control unit 23. In addition, the BSC 109 also comprisesa memory unit 24, which can be used both to store information and tostore executable code which is used during operation of the BSC 109. Theinformation which is stored in the memory unit 24 can, for example, bethe reserved or predefined value or values of the LI field whichindicate the presence of a PLMN ID in an RLC data block.

In addition, information regarding where in an RLC data block to findthe PLMN ID if the LI field in the RLC data block is the reserved orpredefined value which indicates the presence of a PLMN ID is alsosuitably stored in the memory unit 24.

FIG. 4 shows a schematic flow chart of a method 400 for operating amobile station in a Radio Access Network, a RAN, where the RAN isconnected to a core network in which there is one or more Public LandMobile Networks, PLMNs.

As shown in step 405, the method comprises having the Mobile Stationchoose one of said PLMNs when accessing a Packet Switched, PS, domain inthe RAN (e.g. when attaching to the RAN 102 or a Core Network, CN, towhich the RAN is connected), and as indicated in step 410, the method400 comprises including the identity, the ID, of the chosen PLMN in anRLC data block.

Step 420 shows that the method 400 comprises including a reserved orpre-defined value in the length indicator field of the RLC data block asinformation to the RAN of the presence of the ID of the chosen PLMN inthe RLC data block, and step 430 shows that the method comprises sendingsaid RLC data block to the RAN.

In embodiments, the method comprises, as shown in step 415, includingthe ID of the chosen PLMN after the last Upper Layer PDU of the RLC datablock.

In embodiments, the method comprises, as shown in step 425, the use ofan LI value of 123 as the reserved or pre-defined value.

In embodiments, the method comprises, arranging the mobile station touse different RAN modes and to use different reserved or pre-definedvalues to indicate the presence of the ID of a chosen PLMN depending onthe RAN mode used.

FIG. 5 shows a schematic flow chart of a method 500 for operating aControl Node, e.g. Base Station Controller, a BSC, in a Base StationSystem, a BSS. Other examples of such control nodes include an LTEeNodeB and a WCDMA RNC, Radio Network Controller.

The control node is arranged to be part of a Radio Access Network, aRAN, which is connected to a Core Network, a CN, which comprises one ormore Public Land Mobile Networks, PLMNs.

As shown in step 505, the method 500 comprises receiving Radio LinkControl, RLC, data blocks from a mobile station in a cell in the RAN,and also comprises, as shown in step 510, inspecting the LengthIndicator field in said RLC data blocks, and, step 515, identifying areserved or pre-defined value which indicates that the RLC data blockcomprises the ID of one of said PLMNs which has been selected by the UE.

As shown in step 520, the method 500 comprises finding and extractingsaid ID in the RLC Data block, and, step 525, forwarding the ID to anode in the RAN or in the CN.

According to embodiments of the method 500, the node to which the PLMNID is forwarded to is an SGSN.

According to embodiments of the method 500, the PLMN ID is found afterthe last Upper Layer PDU of the RLC data block in which the PLMN ID isincluded.

In embodiments, the method 500 comprises handling more than one RANmode, and handling different reserved or pre-defined values whichindicate the presence of the ID of a chosen PLMN, depending on RAN mode.

Embodiments of the invention are described with reference to thedrawings, such as block diagrams and/or flowcharts. It is understoodthat several blocks of the block diagrams and/or flowchartillustrations, and combinations of blocks in the block diagrams and/orflowchart illustrations, can be implemented by computer programinstructions. Such computer program instructions may be provided to aprocessor of a general purpose computer, a special purpose computerand/or other programmable data processing apparatus to produce amachine, such that the instructions, which execute via the processor ofthe computer and/or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the blockdiagrams and/or flowchart block or blocks.

These computer program instructions may also be stored in acomputer-readable memory that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablememory produce an article of manufacture including instructions whichimplement the function/act specified in the block diagrams and/orflowchart block or blocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer-implemented process such that theinstructions which execute on the computer or other programmableapparatus provide steps for implementing the functions/acts specified inthe block diagrams and/or flowchart block or blocks.

In some implementations, the functions or steps noted in the blocks mayoccur out of the order noted in the operational illustrations. Forexample, two blocks shown in succession may in fact be executedsubstantially concurrently or the blocks may sometimes be executed inthe reverse order, depending upon the functionality/acts involved.

In the drawings and specification, there have been disclosed exemplaryembodiments of the invention. However, many variations and modificationscan be made to these embodiments without substantially departing fromthe principles of the present invention. Accordingly, although specificterms are employed, they are used in a generic and descriptive senseonly and not for purposes of limitation.

The invention is not limited to the examples of embodiments describedabove and shown in the drawings, but may be freely varied within thescope of the appended claims.

The invention claimed is:
 1. A mobile station for use in a Radio AccessNetwork (RAN), wherein the RAN is connected to a core network thatcomprises one or more Public Land Mobile Networks (PLMNs), the mobilestation comprising: an interface to communicate with the RAN using RadioLink Control (RLC) data blocks; a processor to: choose one of said PLMNswhen accessing a Packet Switched (PS) domain in the RAN; include anidentity (ID) of a chosen PLMN in an RLC data block; and include areserved or pre-defined value in a length indicator field of the RLCdata block to inform the RAN that the RLC data block contains the ID ofthe chosen PLMN, wherein a value of 123 or 62 is used as the reserved orpre-defined value included in the length indicator field.
 2. The mobilestation of claim 1, wherein the ID of the chosen PLMN is included aftera last Upper Layer PDU of the RLC data block.
 3. The mobile station ofclaim 1, wherein the mobile station is operable using different RANmodes, and the reserved or pre-defined value used to inform the RAN thatthe RLC data block contains the ID of the chosen PLMN is differentdepending on which of the RAN modes is used.
 4. The mobile station ofclaim 3, wherein the reserved or pre-defined value is different for RANmodes GPRS and EGPRS.
 5. A Control Node arranged to be part of a RadioAccess Network (RAN), wherein the RAN is connected to a Core Network(CN) that comprises one or more Public Land Mobile Networks (PLMNs), theControl Node comprising: an interface to receive Radio Link Control(RLC) data blocks from a mobile station in a cell in the RAN; aprocessor to: inspect a length indicator field in an RLC data block;identify a reserved or pre-defined value in the length indicator field,wherein the reserved or pre-defined value indicates that the RLC datablock contains an ID of a PLMN selected by the mobile station from theone or more PLMNs, wherein a value of 123 or 62 is identified as thereserved or pre-defined value in the length indicator field; find andextract said ID in the RLC data block; and forward the ID to a node inthe RAN or in the CN.
 6. The Control Node of claim 5, wherein said nodeto which the ID is forwarded to is a Serving GPRS Support Node (SGSN).7. The Control Node of claim 5, wherein the processor is furtheroperable to find said ID after a last Upper Layer PDU of the RLC datablock in which the ID is included.
 8. The Control Node of claim 5,wherein the Control Node is operable using more than one RAN mode, andthe reserved or pre-defined value used to indicate that the RLC datablock contains the ID of the selected PLMN is different depending onwhich RAN mode is used.
 9. The Control Node of claim 8, wherein thereserved or pre-defined value used to indicate that the RLC data blockcontains the ID of the selected PLMN is different for RAN modes GPRS andEGPRS.
 10. The Control Node of claim 5, wherein the Control Node is aGSM Base Station Controller (BSC) arranged to be part of a Base StationSystem (BSS).
 11. The Control Node of claim 5, wherein the Control Nodeis an LTE eNodeB.
 12. The Control Node of claim 5, wherein the ControlNode is a WCDMA Radio Network Controller (RNC).
 13. A method performedby a mobile station comprising an interface and a processor, wherein themobile station is connected to a Radio Access Network (RAN) and the RANis connected to a core network comprising one or more Public Land MobileNetworks (PLMNs), the method comprising: choosing one of said PLMNs whenaccessing a Packet Switched (PS) domain in the RAN; including anidentity (ID) of the chosen PLMN in an RLC data block; including areserved or pre-defined value in a length indicator field of the RLCdata block to inform the RAN that the RLC data block contains the ID ofthe chosen PLMN, wherein a value of 123 or 62 is used as the reserved orpre-defined value in the length indicator field; and sending the RLCdata block to the RAN.
 14. The method of claim 13, wherein the ID of thechosen PLMN is included after a last Upper Layer PDU of the RLC datablock.
 15. The method of claim 13, wherein the mobile station isoperable using different RAN modes, and the reserved or pre-definedvalue used to inform the RAN that the RLC data block contains the ID ofthe chosen PLMN is different depending on which RAN mode is used. 16.The method of claim 15, wherein the reserved or pre-defined value isdifferent for RAN modes GPRS and EGPRS.
 17. A method performed by aControl Node comprising an interface and a processor, wherein theControl Node is part of a Radio Access Network (RAN) and the RAN isconnected to a Core Network (CN) comprising one or more Public LandMobile Networks (PLMNs), the method comprising: receiving Radio LinkControl (RLC) data blocks from a mobile station in a cell in the RAN;inspecting a length indicator field in an RLC data block; identifying areserved or pre-defined value in the length indicator field, wherein thereserved or pre-defined value indicates that the RLC data block containsan ID of a PLMN selected by the mobile station from the one or morePLMNs, wherein a value of 123 or 62 is identified as the reserved orpre-defined value in the length indicator field: finding and extractingsaid ID in the RLC data block; and forwarding the ID to a node in theRAN or in the CN.
 18. The method of claim 17, wherein said node to whichthe ID is forwarded to is a Serving GPRS Support Node (SGSN).
 19. Themethod of claim 17, wherein said ID is found after a last Upper LayerPDU of the RLC data block in which the ID is included.
 20. The method ofclaim 17, wherein the Control Node is operable using multiple RAN modes,and the reserved or pre-defined value used to indicate that the RLC datablock contains the ID of the PLMN selected by the mobile station isdifferent depending on which RAN mode is used.